I am not sure. I would say that my best guess, when I ask myself that question, is, barring the possibility that he is not sure if he 'loves' you, that it could be a fear of exposure, a fear of being vulnerable to the SO. Saying the words can take a guarding wall down and leave you exposed to getting hurt. Takes a lot of trust. It's like when you fall backwards and you are hoping your SO catches you before you hit the ground. Keep telling him you love him, but don't ask him to say the words (be understanding and supportive). He'll say them when the time is right (for him).